The present invention relates to a method of manufacturing a ceramic thick-film printed circuit board, the method which is intended for improvement of printing accuracy by preventing sagging in a printing process for the ceramic thick-film circuit board production.
Recently, reduction in size and weight of electronic components is demanded, arising from the need for their use in cellular telephones and compact office automation equipment. Owing to multi-layering such as a green sheet laminating method or the like, a ceramic thick-film printed circuit board in particular meets the demand for its use for a high-density printed circuit board such as a microchip module (MCM) or a chip size packaging (CSP).
Even in the chip components industry, multi-layering as can be seen in chip capacitors and chip inductors has been employed as the mainstream to meet miniaturization of chip components. Moreover, in the case of chip resistors and the like, the use of high-resolution screen masks and the introduction of paste of good printability in a screen printing process encourage fine printing.
Furthermore, the manufacture of chip components utilizing an intaglio transfer printing technique which is acceptable for fine printing is also commercially practical.
In the case of the manufacture of such a high-density printed circuit board, the multi-layering requires a many cycles of printings. Besides, the number of times an intaglio is used is limited in the intaglio transfer printing technique, so that cost for such materials as intaglios to be used has increased.
The present invention provides a method of manufacturing a printed circuit board, realizing improvement of printed wiring density by preventing printing paste sagging through the formation of an anti-sagging layer on the surface of a substrate prior to a printing step for a ceramic thick-film printed circuit board production.
In a conventional process of manufacturing a thick-film printed circuit board, sagging of the paste during or after printing has caused limitation on the wiring density. For this reason, in cases where an alumina substrate is used as the substrate, a printed pattern with a line width of 100 xcexcm has required a spacing of about 70 to 80 xcexcm. Paste, which readily sags, particularly has required a spacing of 100 xcexcm or more. There is one method proposed as a measure against paste sagging. According to the method, an anti-sagging layer (hereinafter referred to as resin layer) mainly consisting of an organic binder contained in the printing paste is formed on an object to be printied before printing, so that the resin layer absorbs a solvent contained in the paste, thereby increasing viscosity of the paste and preventing the paste from sagging.
The conventional printing process utilizing the resin layer is explained hereinafter with reference to FIGS. 2A-2D and 3A-3D. First, with reference to FIG. 2A, there is explained a method of forming a resin layer by screen printing generally employed as a resin layer forming method.
Ethyl cellulose generally used as an organic binder contained in printing paste is used as a material for a resin layer as shown in FIG. 2A. Ethyl cellulose is dissolved in a solvent such as xcex1-terpineol or the like at a concentration of about 10 wt % to form a anti-sagging paste for use. The anti-sagging paste 23 is placed on screen mask 22 in a manner as shown in FIG. 2A.
Subsequently, anti-sagging paste 23 is printed, using squeegee 24, to form a resin layer on ceramic substrate 21, an object to be printed. FIG. 2B illustrates resin layer 25 being formed on ceramic substrate 21. Here, there has been a problem of mesh marks remaining on the substrate due to poor leveling of paste 23.
Thereafter, as shown in FIG. 2C, printed layers 26 are formed on resin layer 25 by screen printing. Here, depending on the sizes of mesh marks on resin layer 25, there have been cases where a printed pattern has blurred after the printing, thus forming short circuit portion 27, as shown in FIG. 2D.
As a method for preventing the mesh marks remaining on resin layer 25, reducing a viscosity of paste 23 can be conceivably. In this case, however, resin layer 25 per se readily extends off ceramic substrate 21, so that an printing area of resin layer 25 on the substrate has to be reduced.
Problems of the resin layer that arise in the conventional printing process are explained next with reference to FIGS. 3A-3D.
In a case where paste 33 is printed, using screen mask 32 as shown in FIG. 3A, a screen mask of 250 mesh or more is used. In this case, resin layer 35 has a thickness of 4 xcexcm or more when dried.
Subsequently, as shown in FIG. 3B, printed layer 36 is formed on resin layer 35 on ceramic substrate 31. Thereafter, during printed layer 36 is fired, resin layer 35, which is an organic substance, burns out as shown in FIG. 3C, and only fired film 37 is formed. However, when printed layer 36 is formed on resin layer 35 having a thickness of 5 xcexcm or more and then fired, fired film 37 forms film exfoliation 38 as shown in FIG. 3D at its edge.
To reduce the resin layer in thickness to meet against the above problem, when a screen mask as fine as 325 mesh or more is used for printing paste 33, there has been a problem that the mask poorly peels off, thereby producing bumpy surface of resin layer 35 formed.
As described above, the conventional method has had a great many problems connected with the formation of the resin layer intended for the prevention of sagging as well as with the formation of good-quality printed layers.
In other words, the condition of the printed pattern and the appearance thereof after firing have varied according to the molecular weight of ethyl cellulose in the paste for the resin layer, thickness of the resin layer, thickness of the printed pattern and resin layer forming methods, and depending on these conditions, there have arose problems.
The molecular weight of ethyl cellulose to be used for the resin layer particularly exerts a great influence. The fired pattern forms film exfoliation or become deformed unless the thickness of the resin layer is controlled according to the molecular weight. Moreover, the printed pattern may exfoliate in the course of firing unless the molecular weight of ethyl cellulose and the thickness of the resin layer as well as a temperature rising rate are optimally controlled. Furthermore, cracks have formed in the printed pattern, in a case a solvent content of the printing paste is not properly controlled.
In addition, depending on surface roughness of the resin layer, the printed layer may have pinholes or blur.
The present invention aims to provide a method of manufacturing a high-density printed circuit board of good quality. The method optimizes the conditions for formation of a resin layer, a printing step and a firing step on the processes of the manufacturing a ceramic thick-film printed circuit board.
The present invention provides a method of making high-density printing of high quality in a process of manufacture a ceramic thick-film printed circuit board. The method includes forming a resin layer on a substrate before printing process and subsequently printing a pattern on the resin layer. The present invention provides conditions optimizing material for the resin layer, thickness of the resin layer, surface condition thereof, printing conditions and firing conditions.
According to the manufacturing method of the present invention, a printed circuit board densely printed with a satisfactory printed pattern can readily be obtained. The printed pattern is free of problems such as film exfoliation after firing, deformation of the pattern and pinholes.